SOUTHERN BRASÍLIA BELT (SE BRAZIL): TECTONIC DISCONTINUITIES, K-AR DATA AND EVOLUTION DURING THE NEOPROTEROZOIC BRASILIANO OROGENY

Abstract

This paper focuses the tectonic evolution of the southern Brasília belt, with emphasis on the Furnas segment, along the 21°S parallel. The uppermost structural unit (Passos Nappe - PN) comprises a highly deformed metasedimentary succession interpreted as a fragment of the Neoproterozoic passive margin of western São Francisco craton. An inverted metamorphic gradient ranging from greenschist to lower granulite facies of medium to high-pressure regime characterizes the PN as relict of a subduction zone. The External Domain displays a complex imbrication of basement rocks (Archean Piumhi greenstones, a turbiditic graywacke succession and a calc-alkaline granitoid suite) with undated siliciclastic low-grade metasedimentary rocks. The São Francisco Craton (SFC) comprises pre-1.8 Ga basement rocks covered by anchimetamorphic Neoproterozoic carbonatic shallow marine platform deposits of the Bambuí group. The Brasiliano thrust stacking generated a coarse clastic influx of molassic character on the foreland zone of São Francisco Craton, coeval with the exhumation of the External Domain thrust sheets. New K-Ar determinations on mineral separates are presented and interpreted among previous data. The SFC basement rocks display Paleo- to Mesoproterozoic cooling ages. The allochthonous units, in contrast, display K-Ar ages within the 560-675 Ma range. Brasiliano thrust stacking is therefore interpreted to have taken place onto a “cold” São Francisco craton foreland, in a thin-skinned style, as basement rocks were not heated enough to have their K-Ar systems reset during the allochthony.